Quasi‐Homoepitaxial Growth of Highly Strained Alkali‐Metal Ultrathin Films on Kagome Superconductors

Abstract Applying lattice strain to thin films, a critical factor to tailor their properties such as stabilizing a structural phase unstable at ambient pressure, generally necessitates heteroepitaxial growth to control the lattice mismatch with substrate. Therefore, while homoepitaxy, the growth of thin film on a substrate made of the same material, is a useful method to fabricate high‐quality thin films, its application to studying strain‐induced structural phases is limited. Contrary to this general belief, here the quasi‐homoepitaxial growth of Cs and Rb thin films is reported with substantial in‐plane compressive strain. This is achieved by utilizing the alkali‐metal layer existing in bulk crystal of kagome metals AV3Sb5 (A = Cs and Rb) as a structural template. The angle‐resolved photoemission spectroscopy measurements reveal the formation of metallic quantum well states and notable thickness‐dependent quasiparticle lifetime. Comparison with density functional theory calculations suggests that the obtained thin films crystalize in the face‐centered cubic structure, which is typically stable only under high pressure in bulk crystals. These findings provide a useful approach for synthesizing highly strained thin films by quasi‐homoepitaxy, and pave the way for investigating many‐body interactions in Fermi liquids with tunable dimensionality.


S1. Estimation of film thickness
The thickness of Cs thin films was estimated by comparing the energy separation of quantum well states (QWSs) in the angle-resolved photoemission spectroscopy (ARPES) results with calculations with varying thicknesses, following a common practice in ARPES studies.For example, when the second-derivative ARPES intensity of Figure 2a1 in the main text is compared with band structure calculations with 6-8 monolayers (MLs) (Figure S1a-c, respectively), the energy difference between the topmost and next QWSs best agrees with the calculation for 7 ML (Figure S1b), whereas there is a clear discrepancy for the calculations of 6 and 8 MLs (indicated by white arrows in Figure S1a,c) (note that the calculated bands in these figures were shifted in energy so that the energy position of the topmost QWS coincides with that of the ARPES data).Based on this comparison, the thickness was estimated to be 7 ML.

S2. Comparison of the size of epitaxial strain
In Table S1, we summarize typical examples of epitaxial thin films in which lattice strain has been successfully introduced.  In th table, the size of strain was calculated by 1− aFilm/aBulk, where aFilm and aBulk are the in-plane lattice constants of epitaxial film and its bulk counterpart.One can see that the size of strain is usually less than 3%.The largest strain is limited to ~5% even with the heteroepitaxial growth technique.Compared with these values, the magnitude of strain, as large as 30%, in Cs thin films on CsV3Sb5 is substantially large.

Figure S1 .
Figure S1.(a-c) Second-derivative ARPES intensity for Cs-deposited CsV3Sb5, reproduced from Figure 2b1 in the main text, together with calculated band structures for 6-ML, 7-ML, and 8-ML fcc Cs, respectively (red curves).White arrows highlight an energy difference between the experimental and calculated QWSs.

S3.
Band structure calculations in the presence of epitaxial Cs overlayerTo discuss how epitaxial Cs films influence the electronic band structure and physical property of CVS, we conducted first-principles calculations of the band structure before and after the formation of epitaxial Cs film (the corresponding slabs are depicted in FigureS2aand S2b, respectively).A comparison of the calculated results in Figure S2c reveals that the CVSderived V 3d bands dominating the band structure around the K and M points are almost unchanged.In addition, while the CVS-derived Sb 5p bands near the G point show a finite energy shift due to the hybridization with the QWSs of the Cs film, their Fermi wave vectors are almost unchanged.These findings suggest very little charge transfer between the epitaxial Cs film and CVS, in sharp contrast to the observation of electron doping from a disordered Cs overlayer to CVS.Since the energy position of the saddle point and the Sb 5p band filling are the key band parameters linked to the CDW and superconductivity in CVS, the formation of the epitaxial Cs overlayer may not alter these properties.

Figure S2 .
Figure S2.(a,b) Slabs of CsV3Sb5 without and with epitaxial Cs overlayer, respectively.(c) Calculated band structures using the slabs in (a,b).

Table S1 :
Size of strain applied to epitaxial thin films.